1. Field of the Invention
The present invention relates to satellite communication systems. In particular, the present invention relates to a satellite communication system that utilizes automatic uplink power control to compensate for signal attenuation.
2. Description of the Related Art
Satellite communication systems typically include a terrestrial hub station that transmits a signal to a space-borne satellite, after which the satellite transmits the signal to a terrestrial receiver station. Signals transmitted from a terrestrial station and received by a satellite are referred to as uplink signals, and signals transmitted from a satellite and received by a terrestrial station are referred to as downlink signals. These signals are often subject to attenuation, also referred to as path loss or fading, due to atmospheric factors such as rain, water vapor, clouds, and fog along the wave propagation paths of the signals. In such instances, it is desirable to compensate for signal attenuation in order to maintain a constant energy per bit to noise power spectral density ratio (Eb/N0), and accordingly a constant signal to noise ratio (SNR), at the receiver. A purpose of maintaining a constant Eb/N0 is to ensure signal reliability while optimizing the transmission power of the system. In instances in which attenuation due to rain occurs specifically along an uplink path, a constant SNR at the receiver may be achieved by maintaining a constant flux density at the satellite. One such system is described by U.S. Pat. App. Pub. No. 2008/0274960 to Laufer (US 2008/0274960), the entirety of which is incorporated herein by reference.
To compensate for attenuation, prior art systems have employed automatic uplink power control (AUPC), in which the output power on the uplink signal is adjusted to maintain a constant Eb/N0 at the receiver. Automatic uplink power control is particularly useful at frequencies such as the Ku-band, in which rain is the primary cause of signal attenuation. At much higher frequencies, above 60 GHz, other factors, such as clouds, have a greater effect on signal attenuation than at lower frequencies.
Satellite modems in some point-to-point communications systems have provided transmit power control based on link performance. For example, US 2008/0274960 describes a system in which a hub transmits a signal to a satellite via an uplink, and the satellite relays that signal to a remote station via a downlink. Both the uplink and downlink signals are subject to weather-related attenuation. The overall attenuation (i.e., the sum of both uplink and downlink attenuation) is measured at the remote station and the measurement is transmitted to the hub, where the uplink output power is adjusted to compensate for the measured attenuation. Because compensation in such systems is based on the sum of both uplink and downlink attenuation, the method of power control employed by those systems is inappropriate for point-to-multipoint and other systems in which the attenuation of downlink signals transmitted to multiple receivers may vary significantly. Moreover, attempts to boost the uplink output power to compensate for both uplink and downlink attenuation may saturate satellite transponders.
The system described by US 2008/0274960 is one in which the uplink attenuation is estimated based on the overall attenuation, which is measured at a remote station. The estimate is transmitted from the remote station to a hub, where uplink power control is used to compensate for the estimated uplink attenuation, such that a constant flux density is maintained at the satellite. The downlink attenuation is also estimated based on the overall attenuation, and adaptive coding and modulation (ACM) is used to compensate for downlink attenuation, thereby achieving the desired link performance at the cost of bandwidth. Systems such as the one described by US 2008/0274960 require sufficient time for a hub to transmit a signal to a remote station via satellite and for the remote station to transmit estimated attenuation information back to the hub before the output power of the hub is adjusted to compensate for the estimated attenuation.
U.S. Pat. No. 7,120,392 to Chu et al., the entirety of which is incorporated herein by reference, describes a system that employs a satellite beacon, which is a signal having a frequency different than that of the uplink signal. The beacon is generated by a satellite, and transmitted to a receiver at a hub station. By taking into account the signal strength of the beacon, and the frequency difference between the beacon and the uplink signal, the uplink attenuation is estimated, and uplink power control is used to compensate for the uplink attenuation only. A constant flux density at the satellite is therefore maintained. Systems that employ satellite beacons require that receivers be specially configured to receive the beacon signals.
Accordingly, there exists a need to provide a system in which automatic uplink power control is used to maintain a constant flux density at the satellite, or a constant signal to noise ratio at the remote station, or both, without requiring signal quality reports from a remote station, and without requiring a satellite beacon.